Role of Co-solvents in Biomass Conversion Reactions Using Sub/Supercritical Water

Abstract

Water above its critical point (Tc = 647 K, Pc = 22.1 MPa), which is regarded as supercritical water (SCW), is being given increasing attention as a medium for organic chemistry. This interest in SCW is mainly driven by the search for more “green” or environmentally benign chemical processes. The use of SCW instead of organic solvents in chemical processes offers environmental advantages and may lead to pollution prevention. SCW has been applied in synthetic fuels production, biomass processing, waste treatment, materials synthesis, and geochemistry. However, higher critical parameters of water indicate that the operation process is performed at harsh conditions, thereby increasing cost. Other drawbacks of the use of water as the medium for biomass liquefaction reaction include lower biofuel yield and higher oxygen content. Organic solvents, such as methanol, ethanol, and 2-propanol, have been utilized as co-solvents of SCW to enhance the biofuel yield with lower oxygen content and higher heating value. This paper mainly expounds the basic characteristics of the co-solvent in sub/supercritical water and analyzes the function of the co-solvent in reactions to provide readers with a more comprehensive knowledge of the co-solvent. Based on literature and related studies conducted by our group, systematic analysis about selection and application of co-solvent was conducted.